A drilling machine or the like which bores workpieces using a plurality of tools is provided with a tool stocker holding a plurality of tools each one of which is mounted in a tapered hole of a tool spindle. A necessary machining such as boring is carried out for workpiece while tools are sequentially changed between the tool spindle and the tool stocker according to respective portions of the workpiece to be machined.
An automatic tool changer generally comprises a tool-changing arm having grippers which are provided on both ends thereof respectively and are adapted to simultaneously grip a tool mounted in the tapered hole of the tool spindle and another tool set on a tool change location of the tool stocker respectively. The tool-changing arm is turnable and movable axially. In the tool change, the tool-changing arm is turned by a predetermined angle in the positive direction so that the tools at both tool spindle and tool stocker sides are simultaneously gripped by the grippers provided at both ends of the tool-changing arm, respectively. The tool-changing arm is then moved axially forward so that both gripped tools are taken out of the tool spindle and the tool stocker side respectively. Subsequently, the tool-changing arm is turned 180° and axially retreated so that both tools are attached to the tool spindle and tool stocker sides respectively. The tool-changing arm is finally turned by the predetermined angle in the reverse direction thereby to be returned to an original position (initial position).
The automatic tool changer as described above includes a type in which the turning and the axial movement of the tool-changing arm are mechanically worked together by a cam mechanism in order that the tool-changing arm may be operated quickly and smoothly, as described by patent document 1 (JP-A-2000-126969). In the automatic tool changer as constructed above, part of turning of a tool change shaft and part of the axial movement thereof are overlapped with each other so that a tool-changing time is reduced.
More specifically, the turning of the tool-changing arm starts in the midst of a tool pullout operation so that tools are prevented from interference with the tapered hole of the tool spindle or the like, and a tool inserting operation by the tool-changing arm also starts in the middle of the 180°-turn of the tool-changing arm, so that the tool-changing time is shortened by the overlap of the turning operation and the axial reciprocation of the tool-changing arm.    Patent document 1: JP-A-2000-126969